1. Field of the Invention
The present invention relates to an image reading apparatus and an image forming apparatus.
2. Description of the Related Art
Conventionally, there is known an image reading apparatus for optically reading image information of a document. The image reading apparatus of this type uses a light source extended in a main scanning direction which is orthogonal to a conveying path of the document to irradiate the document with light and receive light reflected by the irradiated document by means of an image sensor to read an image of the document. In recent years, moreover, there is practically used an image reading apparatus of a contact image sensor (CIS) type which uses a light emitting diode (LED) taking a small shape as a light source to form an image on a line sensor through an optical system of same magnification imaging in order to reduce a size of the apparatus.
The image reading apparatus generally carries out a gain adjustment and AD conversion over an analog image signal of a document which is output from an image sensor by an analog front end (AFE) circuit, and performs a predetermined image processing through a digital image processing circuit and then stores the signal in a frame memory. In order to enhance tone reproducibility (resolution) of a digital image signal subjected to the AD conversion, thereby obtaining an image of high quality, it is desirable to carry out such an adjustment as to enable a maximum utilization of a dynamic range for the AD conversion. The adjustment is generally executed by a method of amplifying an analog image signal within a range in which the digital image signal subjected to the AD conversion is not saturated through the gain adjustment to be performed in a former stage of the AD conversion.
When the analog image signal is amplified by the gain adjustment, however, a noise component of a black level included in the analog image signal or the like is also amplified in the same gain, resulting in reduction in S/N. Therefore, the inventor proposes a method of regulating a light quantity of a light source which can utilize a dynamic range for the AD conversion at a maximum (see Japanese Patent Application laid-open No. 2007-81696).
In the technique described in Japanese Patent Application laid-open No. 2007-81696, a light quantity of a light source is regulated in such a manner that a peak value of an image signal output from an image sensor reaches a target value determined depending on a reflectance of a reference white plate when the reference white plate illuminated at a predetermined light quantity is read. More specifically, it is assumed that the image signal of the reference white plate has a peak value of 150, the target value is 200 and the black level included in the image signal (the output of the image sensor in a light quantity of zero) is 10 when the predetermined light quality is set to be one. In this case, the light quantity of the light source is regulated to be approximately 1.36 times as large as the predetermined light quantity by (200−10)/(150−10)≈1.36.
According to the technique described in Japanese Patent Application laid-open No. 2007-81696, only a signal component of an image signal can be increased within a range in which a digital image signal subjected to the AD conversion is not saturated, and a dynamic range for the AD conversion can be utilized at a maximum. Therefore, it is possible to enhance tone reproducibility of a digital image signal subjected to the AD conversion without deteriorating S/N.
Referring to the technique described in Japanese Patent Application laid-open No. 2007-81696, it is assumed that the black level included in the image signal is constant. However, in the case of a structure in which outputs of sensor chips arranged in a main scanning direction are synthesized to obtain an image signal corresponding to one main scanning line, for example, the image reading apparatus of the CIS type described above, the black level is varied every sensor chip due to an individual difference of an output buffer provided in each of the sensor chips or the like. In the case in which a CMOS sensor is used as the sensor chip, moreover, the black level is varied every pixel because the CMOS sensor has an output buffer for each pixel and each output buffer has an individual difference. For this reason, it is impossible to optimize a light quantity of a light source through an exact application of the technique described in Japanese Patent Application laid-open No. 2007-81696.
A range of a variation in the black level is determined for each sensor chip. By assuming a maximum black level which might be generated, therefore, it is also possible to regulate the light quantity of the light source which applies the technique described in Japanese Patent Application laid-open No. 2007-81696. In this case, however, the dynamic range for the AD conversion cannot be sufficiently utilized effectively. On the other hand, in some cases in which a value of the black level is assumed to be smaller than a value of a maximum black level to regulate the light quantity of the light source, the digital image signal subjected to the AD conversion is saturated, resulting in a deterioration in picture quality when reading a document having a high reflectance.
Therefore, there is a need for an image reading apparatus and an image forming apparatus capable of optimizing a light quantity of a light source irrespective of a magnitude of a black level included in an image signal.